Phototherapy is widely used for the treatment of neonatal hyperbilirubinemia. Jaundiced infants are exposed to light for prolonged periods to lower the serum bilirubin concentration, reduce the need for exchange transfusions and diminish the risk of kernicterus and bilirubin encephalopathy. The goal of this project is to elucidate the photometabolism of bilirubin in the newborn using the jaundiced Gunn rat as an animal model. Bilirubin photometabolites, particularly photoisomers of bilirubin, will be isolted, characterized and identified and their metabolism in vivo will be examined. In addition, the mechanism by which bilirubin is converted enzymatically in the liver to polar conjugates will be investigated using synthetic derivatives and analogues of bilirubin.